I’ve mentioned here before the ongoing research on the menagerie of micro-organisms that call us home; the National Institutes of Health is sponsoring the Human Microbiome Project, which is attempting to identify and classify these organisms (which outnumber our own cells by about 10:1), and their roles. Many of these microbes inhabit out digestive systems, and play a beneficial role in aiding digestion and producing certain micronutrients. Research has shown that a delicate balance is maintained between these microbes and the immune system. Disruption of this balance can lead to some serious health problems; some research even suggests that changes to the microbiome can affect mood, via the central nervous system.

A recent article at Ars Technica reports on some new research that uncovers an additional mechanism that helps maintain the intestinal equilibrium between microbes and the immune system.

Cells in the gut sense when the bacteria get too close, and produce a peptide that kills some of them off when they do. This keeps the space around the cells of the small intestine free of bacteria, which in turn keeps the bacteria from setting off a full-blown immune response.

The primary function of the small intestine is the absorption of nutrients and minerals from food; its inside surface is covered with small, finger-like projections (called villi), which increase the effective surface area available to absorb nutrients. This surface has no intrinsic protection from the very large number of bacteria present in the intestine; the study, which was published in Science [abstract], attempted to discover why there is not, in general, a full-scale immune system response.

The mammalian intestine is home to ~100 trillion bacteria that perform important metabolic functions for their hosts. The proximity of vast numbers of bacteria to host intestinal tissues raises the question of how symbiotic host-bacterial relationships are maintained without eliciting potentially harmful immune responses.

Th researchers found that the innate immune system — that “generic” part of the system that recognizes common features of bacteria in general, rather than specific organisms — uses a signalling protein (called MyD88) to regulate the production of an anti-bacterial protein (called RegIIIγ) that acts to maintain a bacteria-free zone around the surface of the intestine.

Here, we show that RegIIIγ, a secreted antibacterial lectin, is essential for maintaining a ~50-micrometer zone that physically separates the microbiota from the small intestinal epithelial surface.

This image shows the separation; the blue projections on the left are the intestinal villi, and bacteria are green. (Image by Shipra Vaishnava and Laura Hooper.)

This is still not the whole story, because the bacteria-regulating protein (RegIIIγ) is only effective against some bacteria (gram-positive organisms). There is presumably another pathway that keeps gram-negative organisms in check, but that is yet to be elucidated.